The present invention relates to vertical heat treatment apparatus.
As far as apparatus which perform heat treatment such as the oxidation, diffusion and CVD processing for semiconductor wafers are concerned, the vertical type of heat treatment apparatus is replacing the conventional horizontal type thereof since there is little intake of air into the reaction chamber when the wafers are loaded and unloaded, and it is possible to control the growth of natural oxide films.
The following is a brief description of a conventional vertical type of oxidation and diffusion furnace, with reference to FIG. 5. This oxidation and diffusion furnace comprises an inside tube 1a and an outside tube 1b, and a boat elevator 22 loads a wafer boat 21 on which a plural number of wafers W are loaded, from a lower end opening portion 23a to inside the reaction chamber 10 of the conventional type of double-wall structure where an outer periphery is surrounded by a heater. Then, the opening portion 23a is made airtight by a cover 23, and a processing gas is introduced to between the inside tube 1a and the outside tube 1b by a gas introduction tube not shown in the figure, this processing gas is supplied to the plural number of wafers W via a plural number of small holes 1c formed in the top portion of the inside tube 1a, and a required heat treatment is performed with respect to the wafers W while the processing gas is exhausted from the inside tube 1a by an exhaust tube (not shown).
Here, a heat retention cylinder 24 is provided between the wafer boat 21 and the cover 23 so that the heat treatment region inside the reaction chamber is insulated from the outside and so that the lower side of the reaction chamber is protected from the heat of the heat treatment region. As shown in FIG. 6, this heat retention cylinder 24 uses a cylindrical container 25 made of glass in order to prevent contamination, and is configured so that the inside of the container 25 is filled with glass wool 26 having long fibers and an insulation effect. Moreover, the container 25 has an airtight structure and the inside has a reduced pressure.
The reason why the heat retention cylinder 24 has a structure such as this is to permit periodic cleaning since the surface of the container 25 is dirtied by the adhesion of processing gas since it is inside the reaction chamber 1. Having an airtight structure means that a long time is not required for drying since the cleaning liquid is not taken into the inside of the heat retention cylinder 24. In addition, the inside of the reaction chamber must have a reduced pressure to a required degree of vacuum since thermal expansion of the air may cause the heat retention cylinder 24 to break if air at atmospheric pressure was sealed inside and heated to a temperature of around 1000.degree. C. for example.
However, conventional heat retention cylinder 24 are made of glass and so they sometimes crack or break due to impacts sustained during its washing or transportation. The heat retention cylinder 24 is replaced when such cracking can be recognized by eye. Nevertheless, when the cracks are extremely small and cannot be recognized by eye, cleaning fluid is taken into the inside of the heat retention cylinder 24 when it is washed and the water vapor expands when it is brought into contact with the high-temperature atmosphere inside the reaction chamber 1, and causes breaking and destruction of the heat retention cylinder 24.
At the present time, there is no apparatus suitable for the detection of extremely small cracks (so-called microcracks) and changes in the pressure inside the heat retention cylinder 24, and so it is not possible to prevent destruction such as that described above.